Radio frequency identification (RFID) transponders (tags) are usually used in conjunction with an RFID base station, typically in applications such as inventory control, security, access cards, and personal identification. The base station transmits a carrier signal that powers circuitry in the RFID tag when the RFID tag is brought within a read range of the base station. Data communication between the tag and the station is achieved by modulating the amplitude of the carrier signal with a binary data pattern, usually amplitude shift keying. To that end, RFID tags are typically integrated circuits that include, among other components, antenna elements for coupling the radiated field, tuning capacitors to form circuits that resonate at the carrier frequency, rectifiers to convert the AC carrier signal to dc power, and demodulators to extract the data pattern from the envelope of the carrier signal.
If fabricated at sufficiently low cost, RFID tags can also be useful in cost-sensitive applications such as product pricing, baggage tracking, parcel tracking, asset identification, authentication of paper money, and animal identification, to mention just a few applications. RFID tags could provide significant advantages over systems conventionally used for such applications, such as bar code identification systems. For example, a basket full of items marked with RFID tags could be read rapidly without having to handle each item, whereas they would have to be handled individually when using a bar code system. Unlike bar codes, RFID tags provide the ability to update information on the tag. However, the RFID technology of today is too expensive for dominant use in such applications. There are several factors that drive up the cost of RFID tags, such as the size of the silicon integrated circuit and production costs associated with attaching the integrated circuit and external resonant circuit components onto a single substrate.
One method of reducing costs of RFID tags known in the prior art is to provide the relatively large electronic components that make up the resonant circuit of the RFID tag on a substrate on which the integrated circuit is also mounted and connected. Such components include inductor coil antennas, dipole antennas, fractal antennas, tuning capacitors, and conductive traces to interconnect them. The conductor layer is typically printed using conductive ink, formed using silk screening techniques, chemically etched, or stamped in a suitable metal foil and adhered to the substrate.
When appropriate components and conductive patterns are formed on the substrate, the integrated circuit is then mounted and electrically connected using conventional chip attachment methods.
One conventional technique known in the prior art for forming the antenna on the substrate and making the attachment of the integrated circuit is illustrated with FIG. 1 and FIG. 2.
FIG. 1 is a top view of a prior art structure. Inductor 3 is formed on a substrate 1. Inductor 3 has an inner terminal 7 and an outer terminal 11. Integrated circuit 5 is mounted upside down on substrate 1, such that the conductive contact pads on substrate 1 align with contact pads on integrated circuit 5.
FIG. 2 illustrates a cross sectional view of the prior art, using the same numerical markers for the same elements as in FIG. 1. Connection to integrated circuit 5 is achieved by mounting the integrated circuit upside down so that pads 27 and 28 on integrated circuit 5 align with contact pad 7 and 9 on substrate 1, respectively.
Referring again to FIG. 1, since inductor 3 generally includes several loops that are larger than integrated circuit 5, it becomes necessary to route a conductor trace 13 from the outer terminal 11 of coil inductor 3 to a contact pad 9 on the substrate in the center of coil inductor 3 in order that pads 7, 9 on substrate 1 for both antenna terminals are sufficient closely spaced to align with pads 27, 28 on integrated circuit 5. In order that conductor 13 does not short the conductor traces that makes up inductor coil 3 where the conductors intersect, conductor 13 must be formed on a second conductive layer.
FIG. 2 illustrates one technique known in the prior art wherein conductor 13 is formed on the back surface of substrate 1, making connections 15, 16 through openings in the substrate in order to connect the conductors on the two sides of the substrates.
FIG. 3 illustrates another technique known in the prior art. Conductor 13 is formed on a first conductive layer on the substrate 1. Dielectric layer 19 is formed on the first conductive layer. A second conductive layer is formed on dielectric 19, appropriate openings 17, 18) made in dielectric 19 in order to connect conductors between the two conductive layers.
Whether forming a second conductive layer on the back surface of the substrate or forming a second conductive layer on top of a dielectric formed on a first conductive layer on the substrate, significant production costs are associated with having to form and pattern a second conductivity layer.